Email messages are a very common method for discussion among a group of people. Usually, one person composes a message and sends it to a number of other people. One or more recipient can then reply to this message and thus form an email thread.
Several email clients deal with conversation detection by detecting email threads. The Internet message format RFC 2822 (see http://www.faqs.ord/rfcs/rfc2822.html) is a common standard that specifies a syntax for text messages within the framework of email systems. It does not provide a precise definition of an email thread, but instead defines how an email thread can be detected using structural attributes embedded in email messages.
FIG. 1 is an example of an email thread 100 and the attributes that can be used to detect it. This thread 100 contains a series of three messages 101, 102, 103 that compose a conversation thread between John and Mary. John first sends a message 101 to Mary, who then replies 102 to John's message, carbon copying (CCing) Bob to notify him about the discussion. In addition, Mary also modifies the subject line of her reply to welcome Bob. John then replies 103 to Mary's reply message, quoting her original message in the message content. Notice the “In-Reply-To”, “References”, and “Message-ID” attributes, which can be followed to detect the thread.
Thread detection based on structural attributes has some challenges:                Not all email clients support the structural attributes required for detecting email threads.        When the thread is broken, some systems may fail to detect the entire email thread. Say, for example, message m3 is a reply to m2, which is a reply to m1, and m2 is deleted. Systems that rely on the “In-Reply-To” attribute will fail to detect that thread.        To continue a conversation, one may compose a new message rather than replying to an existing one. In most email clients, composing a new message initiates a new email thread. Therefore, the new message will not have any structural relationship with the original thread.        One may reply to a message while modifying the recipients (as Mary did in FIG. 1, CCing Bob in her reply), changing the subject of the conversation (as Mary did), or changing the topic (as John did in his reply to Mary). This may indicate the initiation of a new conversation. However, the structural attributes embedded in the message as a result of the reply command will still link the newly composed message and the previous ones.        